Shaayrat lies in Syria's central Homs province, north of several towns where government forces and allied militia backed by Russian air strikes have been fighting the Islamic State jihadist group.

Late last month, regime forces recaptured the town of Maheen after IS had overrun it on November 1.

Government troops have fought to edge closer to the historic town of Palmyra, about 130 km (80 miles) east of Shaayrat and held by IS since May.

Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, said Russia was "building new runways at the Shaayrat airport and reinforcing its surroundings in order to use it soon for operations" in Homs province.

Abdel Rahman said Russian helicopters were already using the nearby T-4 military airport to strike IS targets in Palmyra.

"Syrian regime forces are about three kilometres (less than two miles) from Palmyra and are advancing from the south and west with air cover by Russian helicopters," he said.

The troops have also reached the edges of Al-Qaryatain, a mixed Christian-Muslim village in Homs province that IS seized in August.

The military source said Russian and Syrian aircraft had conducted at least 40 strikes in and around Al-Qaryatain in the past 24 hours.